|Re: Calculator memory sizes|
Message #14 Posted by Eric Smith on 31 Jan 2004, 4:35 p.m.,
in response to message #13 by Andrés C. Rodríguez (Argentina)
Just curious: Where did the early financials (HP 80 & 70) keep the financial data (N,I,PV,PMT,FV)for their primitive TVM solvers. As you state, there is no RAM...
The RAM size for the HP-70 should not have been listed in the table, as I have no information on it. I've been reluctant to disassemble my HP-70 as the back label is pristine. On the other hand, one column of keys is not working, so I need to repair it anyhow.
In the HP-80, some of the chips are in a hybrid module, so I can't tell exactly what chips are present. I was basing the information in the table on U.S. Patent 3,863,060. However, the code in the patent may not be the same as that of the final product. For instance, the source code listing in the patent has 1.75K of code (seven ROM chips), which the article "Microcode: Electronic Building Blocks For Calculators" (Hewlett Packard Personal Calculator Digest, Vol. 3, pp. 4-6, 1977) states that the HP-80 contains two quads worth of ROM (2 Kwords, which would be eight of the 256-word chips).
The source code published in the patent simply keeps the financial data in the stack, and will produce incorrect answers if you perform intermediate calculations using more than two stack levels between the entry of two financial variables. I do not have an actual HP-80 close at hand, but I suspect that this behavior is not present in the released software. It is reasonable to expect that the real HP-80 contains an extra RAM chip, and that it may well have more ROM code than listed in the patent in order to deal with the RAM variables. I won't know for certain until I dump the HP-80 ROM code. So for the present, I have removed the RAM information from the HP-80 table entry as well, and added a footnote about the 1.75K vs. 2K question.
The HP-80, HP-80, HP-22, and HP-27 have what I call "old-style TVM", which differs significantly from the TVM capabilities of modern financial calculators in two ways:
1) Only three variables are used as input to the old-style TVM, with one or two remaining variables calculated as output. See the table on page 35 of the HP-70 Owner's Handbook (p. 19 of the PDF file on the MoHPC CD-ROMs) for a list of the valid input and output combinations.
2) The old-style TVM does not use the sign convention to distinguish incoming and outgoing cash flows. The PV, PMT, and FV values are normally all positive.
The HP-92 was the first calculator to introduce the new-style TVM used on all subsequent HP financial calculators (HP-37E, HP-38E/C, HP-12C, etc.). The new-style TVM solves a single five-variable equation (requiring four known values and one unknown), and uses the sign convention for the direction of cash flows.